Environmental-mediated relationships between tree growth of black spruce and abundance of spruce budworm along a latitudinal transect in Quebec, Canada

Changes in tree growth and insect distribution are projected due to climate warming. The expected effectsof climate change on forest disturbance (e.g., insect outbreak) regime call for a better insight into thegrowth responses of trees to varying environmental conditions over geographical regions in eastern NorthAmerica. In this study, the effects of a latitudinal thermal gradient and spruce budworm (SBW) outbreakson the tree growth of black spruce (Picea mariana Mill.) were investigated along a 400 km transect from48◦N to 51◦N across the continuous boreal forest in Quebec, Canada. Time series data were analyzed tosynchronize climatic factors (temperature and precipitation trends), insect dynamics (SBW populationfrequency) and tree growth (ring-width chronology). Radial growth resulted as being synchronized withclimate patterns, highlighting a positive effect of maximum temperatures on tree growth, especially in thenorthernmost site. Increasing temperatures and precipitation had a more positive effect on tree growthduring epidemic periods, whereas the detrimental effects of SBW outbreaks on tree growth were observedwith climate patterns characterized by lowered temperature. The lag between time series, synchronyand/or frequency of synchrony between tree growth and SBW outbreak were considered in order tolink the growth of host trees and the dynamics of insect populations. The proposed analytical approachdefined damage severity on tree growth in relation to population dynamics and climate fluctuations atthe northern distribution limit of the insect. Overall, a decline in tree growth was observed in these borealforests, due to SBW outbreaks acting in combination with other stress factors.